Cellular communications networks are well known for both mobile and fixed subscribers. In cellular networks the coverage area is divided into cells, each cell is served by a base station or base site which allocates a frequency or group of frequencies which define communication channels between the subscriber and base station. The number of frequencies available to a cellular network is limited; so the frequencies are re-used over different cells to make maximum use of these frequencies while at the same time maintaining an acceptable level of interference between neighboring cells operating on the same frequency. Cells can be divided into sectors, each sector being allocated a particular frequency or group of frequencies, with reuse of the frequencies. In some conventional mobile subscriber sectored networks, frequencies are allocated such that the same frequency is given to corresponding similarly aligned sectors in each base site. While this reduces the level of interference from adjacent (first pattern repeat) cells, inference still occurs with cells beyond the adjacent cell-second, third pattern repeat and so on interference. Such arrangements are a particular problem in FWA (fixed wireless access) networks where subscribers have high gain narrow beamed directional antennas, because the directional antenna is still aligned with co-frequency sectors in adjacent (first pattern repeat) and more distant (second, third . . . pattern repeat) cells.
There have been attempts to reduce co-frequency interference by varying the direction of sectors in some cells with respect to other cells such that sectors having the same frequency or frequency group are mis-aligned. By rotation of some cell sectors with respect to other cell sectors, direct co-frequency interference from first pattern repeat cells can be reduced.
It has also been known to change the polarization of two adjacent co-channel sectors on an ad hoc basis to overcome severe and localised cases of co-channel interference.
Another method of reducing interference is to increase the number of frequencies or frequency groups allocated to each cell by increasing the number of sectors. In this way each sector is narrower and will therefore be less exposed to co-frequency sectors of adjacent cells, and for directional subscribers with suitable sector rotation the cell distance between direct interfering sectors can be increased. The number of frequencies or frequency groups allocated to each cell is known as the frequency reuse factor N which is a product of the base re-use factor Nb and the sector re-use Ns. The frequency re-use factor in GSM type mobile systems is typically 12–48. Generally the higher the frequency re-use factor N, the lower the co-frequency interference and hence the better the carrier to interference ratio (C/I). However the high frequency re-use factors typical in mobile systems reduces the capacity of the system in that less frequencies are available per base.
Most prior art systems are concerned with serving subscribers who are equipped with omnidirectional antennas such as mobile phones, which receive signals equally from all directions. The allocation of frequencies to base stations in these systems is therefore typically restricted to prevent strong unwanted interfering signals from first pattern repeat cells. For directional receivers as used typically in FWA systems and some mobile applications, different reuse patterns may be more effective, exploiting the fact that sectors can reuse frequencies with less spatial separation provided they are not facing the directional receivers of other sectors, in other words if there is directional misalignment. One known arrangement shown in U.S. Pat. No. 6,405,044 involves maintaining directional mis-alignment and polarization differences across the network between sectors carrying common frequencies in order to reduce co-frequency interference.
Another known arrangement involves using a number of different reuse patterns for different channels. This is called a tiered approach. Some channels are arranged in a first tier which uses an aggressive reuse pattern with a low reuse factor N, to achieve high capacity at the expense of poor CDF (coverage distribution function) profile. To enhance the C/I for the areas of poor coverage, the remaining channels are in a second tier with a reuse pattern with a higher reuse factor N, which has better CDF profile, but at the expense of capacity. When a base station allocates a channel to a user, it tries to allocate a channel in the first tier, but if the C/I is too low, it may move the user to a channel in the second tier. Such tiering is known for “threshold” type second generation 2G cellular networks where a minimum C/I is needed to initiate a link at all.
A different approach proposed as an evolution of 3G networks, can use adaptive modulation and coding (AMC) of the channels to provide higher data rates over some areas having a higher C/I, though when there are users in the areas of weaker signal strength, there will be a lower data rate available.